Photovoltaic cells such as silicon or III-V compound solar cells are capable of converting solar radiation into usable electrical energy. The electrical energy conversion occurs as a result of what is well known in the solar cell field as the photovoltaic effect. Solar radiation impinging on a solar cell is absorbed by the semiconductor layer, which generates electrons and holes. The electrons and holes are separated by a built-in electric field, for example, a rectifying junction such as a PN junction in the solar cell. The electrons flow towards the N-type region and the holes flow towards the P-type region. The separation of the electrons and holes across the rectifying junction results in the generation of an electric current known as the photocurrent and an electric voltage known as the photovoltage.
Photovoltaic researchers have been investigating various paths toward the generation of electricity from sunlight on an economic basis which can compete with conventional means of generating electricity. The research has focused mainly on two alternatives for economically making electricity from solar cells. For the first alternative, researchers are trying to make low-cost non-crystalline solar cells, such as amorphous silicon cells, and thereafter deploy the cells as large area flat plate arrays. For the second alternative, researchers use a plastic lens as the large area collector in combination with smaller but higher efficiency solar cells. The lens (or array of lenses) focuses the sunlight onto the small area single crystal solar cell (or array of solar cells).
This invention focuses on improved high efficiency single crystal solar cells for the second alternative although if the materials costs were lowered, the cell could be used in the first alternative. To date, the solar cells with the highest conversion efficiencies have been fabricated from the III-V compound semiconductor material, GaAs.
Multicolor solar cells, i.e., cells which absorb light at two or more wavelengths in two or more materials, promise still higher conversion efficiencies. Multicolor solar cells have been described in various U.S. Pat. Nos. such as 4,017,332; 4,179,702; and 4,128,733; and in various technical publications such as the Fifteenth IEEE Photovoltaic Specialists Conference, 1981, pp. 1289-1293.
Multicolor solar cells are formed from various semiconductors each containing a light sensitive junction and each semiconductor material is sensitive to a different portion of the solar spectrum. The simplest, lowest cost multicolor cell is formed by growing these various layers in succession as a stack of single crystal films on a single crystal wafer.
Devices have been fabricated with Al.sub.1-x Ga.sub.x As.sub.y Sb.sub.1-y, Al.sub.1-x-y Ga.sub.x In.sub.y As, or Ga.sub.1-x In.sub.x As.sub.y P.sub.1-y material systems employing vertical lattice matching with, for example, GaAs.sub.1-x Sb.sub.x, Ga.sub.1-x In.sub.x As, and Al.sub.1-x Ga.sub.x As.sub.1-y Sb.sub.y, Ga.sub.1-x In.sub.x P, respectively.
The growth systems used to fabricate these devices have employed rapid layer growth via liquid phase epitaxy systems (LPE) or metal organic chemical vapor deposition systems (CVD) operating at 1 atmosphere total pressure.
The Al containing compounds exhibit stability problems when exposed to ambient conditions. Al has an affinity for oxygen and carbon incorporation. This makes oxygen and carbon impurity incorporation a problem during film growth and it makes the final devices less stable in outdoor deployment in moist air.
Growth of the Ga.sub.1-x In.sub.x As.sub.1-y P.sub.y compound by metal organic CVD has experienced chemical problems resulting from the indium metal organic transport agent, triethyl indium (TEIn). One problem is that TEIn has a very low vapor pressure, making it difficult to supply to the growth zone. A second problem with TEIn is that it reacts prematurely at room temperature with AsH.sub.3 and PH.sub.3. The resultant compound, formed by alkane elimination, is not volatile. These problems make it difficult to work with the GaInAsP system.
U.S. Pat. No. 4,278,474 describes using Si, GaAsP and GaAs/GaAsP superlattices. However, this system suffers from two problems. First, large lattice mismatch, and second, very large thermal expansion coefficient mismatch. The lattice mismatch is about 4%. Superlattices have been fabricated to solve the first problem. However, the problem of thermal mismatch is more difficult to solve. The GaAsP layer grown on Si is in thermal equilibrium at the growth temperature; when cooled, it shrinks much more than does the silicon substrate. The result is a cracked GaAsP layer which ruins the solar cell.
Thus, it would be highly desirable to have a ternary III-V compound semiconductor material system which does not contain elements which cause the fabricated layer to react with ambient weather conditions which shorten the lifetime of the photovoltaic device, e.g., Al.sub.1-x Ga.sub.x As. Furthermore, it would be desirable to have ternary or quaternary III-V layers which lattice match to within .+-.1% and do not require superlattices, e.g., U.S. Pat. No. 4,278,474, to avoid the mismatching of the active layers of a multicolor photovoltaic cell. In addition, it would be a desirable option to have a multicolor cell which can incorporate very thin transition layers to avoid or reduce the mismatch between the active layers. It would also be desirable to have a multicolor cell which was not restricted to selecting materials that lattice match only a single lattice constant, wherein the layer's composition is fixed by the need to grow vertically up a material system from a III-V compound alloying on the V side, e.g., GaAs.sub.1-x Sb.sub.x, to a III-V compound alloying on the III and V sides, e.g., Al.sub.1-x Ga.sub.x As.sub.1-y Sb.sub.y. It would also be desirable to have a growth process which permits the multiple layers to be fabricated at lower temperatures of about 500.degree. C. to about 650.degree. C. and at lower growth rates of about from 1 .mu.m/m to about 10 .mu.m/m than are required by LPE. It would also be desirable to have a multicolor cell system wherein the layers are alloyed on the V side and with V compounds which can be supplied in excess, because of their volatility, without affecting the semiconductor layer. It would be desirable to avoid a layer fabricated from materials, e.g., triethyl indium (TEIn) which prematurely react with the other compounds, e.g., AsH.sub.3 and PH.sub.3, needed to form the semiconductor layer.